Fibre chemistry
A monomer is a small molecule. When thousands of monomers join in a chain, the result is a polymer. Every fibre is made of polymers.
- Cotton, linen, rayon: polymer of glucose called cellulose. It has many –OH (hydroxyl) groups that love water.
- Wool, silk: protein. Chains of amino acids with –NH₂ (amino) and –COOH (carboxyl) groups. They can carry plus (+) or minus (−) charge depending on the pH.
- Nylon: a polyamide, like a protein but man-made. It has amino groups at chain ends.
- Polyester: made by joining an acid and an alcohol. It has almost no reactive groups and does not like water.
- Acrylic: carbon chain with some negative sites added so it can take dye.
Part of a fibre is crystalline (chains packed in order, dye cannot enter) and part is amorphous (loose chains, dye can enter). Heat and water open up the amorphous part.
Chemistry of dyeing
Dyeing is three steps: the dye moves from the water to the fibre surface, enters the fibre, and then fixes (bonds) there. The way it bonds depends on the dye class:
- Reactive dye + cotton: forms a strong covalent bond with the –OH of cellulose in alkaline conditions. Very good wash fastness.
- Acid dye + wool, silk, nylon: dye is negative (−), the fibre is positive (+) in an acid bath, so they attract by an ionic bond.
- Disperse dye + polyester: dye is not soluble in water. Above 120 °C it dissolves inside the fibre, like sugar in tea.
- Basic (cationic) dye + acrylic: dye is positive (+) and sticks to the negative (−) sites of acrylic by an ionic bond.
- Vat dye + cotton (e.g. indigo): made soluble by a chemical, enters the fibre, then air turns it back to insoluble colour that is trapped.
Affinity is how strongly a dye is drawn to a fibre. High affinity = more dye taken up and a firmer hold.
What decides how much dye goes in
Temperature. Heat makes molecules move faster and opens the fibre, so dye enters faster. Polyester needs a lot: about 130 °C under pressure.
pH. pH decides the charge. Wool dyed with acid dye needs pH about 4. Reactive dye on cotton needs alkali, pH about 11. A wrong pH gives pale or patchy colour.
Salt. Common salt in a cotton bath pushes dye out of the water onto the fibre, because both dye and cotton carry a slight negative charge and salt reduces the push-away.
Time and liquor ratio. More time or less water per kg of cloth gives a deeper shade, up to the point where the fibre is full.
Chemicals for fibres and dyeing
A dye house uses many helper chemicals, each with a job:
- Alkali (soda ash, caustic soda): cleans cotton (scouring) and fixes reactive dyes.
- Acid (acetic acid, formic acid): sets the right pH for wool, silk and nylon.
- Salt (sodium chloride, Glauber's salt): pushes dye onto cotton.
- Mordant (alum, iron salt): a metal salt that links natural dye to the fibre and fixes the colour.
- Bleach (hydrogen peroxide): removes natural colour to give a clean white base.
- Reducing and oxidising agents: make vat dyes soluble, then fix them again.
- Wetting agents and detergents (surfactants): help water enter the cloth and remove oil and dirt.
- Levelling agents: slow the dye so the colour goes on evenly.
These chemicals must be handled safely and the waste water treated before it goes into rivers.
Try it
In the 3D, choose cotton with an acid dye and see how little dye is taken up. Then choose the matching dye. Change the pH and the temperature and see how the dye uptake changes. At home: dip a piece of cotton and a piece of polyester (an old white shirt label) in strong turmeric or tea water and compare how much colour each keeps after a wash.
Key formulas and definitions
- Monomers → (join) → polymer chain → fibre
- Cotton: cellulose (–OH) | Wool, silk: protein (–NH₂, –COOH) | Polyester: polyester | Acrylic: carbon chain with − sites
- Match: Reactive–Cotton, Acid–Wool/Silk/Nylon, Disperse–Polyester, Basic–Acrylic
- Best pH: reactive ~11, acid ~4; best temperature: polyester disperse ~130 °C
Worked examples
1. Why can wool be dyed with an acid dye in an acid bath?
In acid the –NH₂ groups of wool pick up a hydrogen ion and become positive. The acid dye is negative, so the two attract and bond ionically.
2. Which dye class would you choose for a polyester sports shirt, and why is the bath heated to about 130 °C?
Disperse dye. Polyester has tightly packed chains and no reactive sites. At about 130 °C the chains loosen and the dye dissolves inside the fibre.
3. Why is salt added when dyeing cotton with a reactive dye?
Cotton and the dye both carry a small negative charge and push each other away. Salt cuts down this push, so more dye moves onto the fibre.
4. A dyer used a reactive dye on wool at pH 11 and got a weak colour. Give two reasons.
Reactive dyes are designed for the –OH of cotton, so the match is poor. Also wool is damaged by strong alkali and loses its sites. Use an acid dye at pH about 4.
5. Why is a mordant needed for many natural dyes?
Many natural dyes have low affinity. A metal salt (mordant) links the dye molecule to the fibre, so the colour stays and often turns deeper.
Common mistakes
- Using the same dye for every fibre. Each fibre class needs its matching dye.
- Thinking a higher temperature is always better. Wool is damaged by too much heat and alkali.
- Mixing up the charge: in acid wool becomes positive and an acid dye is negative.
- Confusing a mordant with a dye. A mordant is a helper that fixes the dye; it is not the colour.